Thursday, May 10, 2007

Nerite snails: Let's talk about shells

Three NAKED facts

Shells protect snails from all kinds of dangers.

Thoughtful question: Can you think of an animal that might want to eat this snail? How about other things that can harm the snail?

Thoughtful question: Let's look at its shape. What does it remind you of? How does this shape help protect the snail?

NF#1: Shell shape: Round and smooth like half a marble, this streamlined shape allows the snail to avoid being washed off the rocks when it is splashed by waves and swept by currents.

It also makes it difficult for a crab to get a grip on the slippery smooth shell, or to crack it. Like a marble, the snail is likely to slip out of the crab's pincers and bounce and roll away!

NF#2: Shut the door! When it is in danger, the snail can quickly retract into its tough thick shell. It also hides in its shell to stay wet, when it is hot and dry outside.

To cover the hole in the shell, it has a tough door (operculum). In nerites,
the operculum has an internal peg to lock firmly in place. This makes it difficult for a crab to stick a pincer in and dig out the snail.

NF#3: Colours: Although some shells are colourful, these snails are hard to spot as they blend well with the colourful rocks where they are found.

Thoughtful comment: Shells can be very pretty. Many people collect pretty shells and don't realise how beautiful the living animal is. See, this snail has a striped body! And such cute long tentacles.

Did you know that shells sold in souvenir shops are taken from living snails? The snails are killed for their shells! So please don't buy shells.

Hints for Naked Hermit Crabs

Where to find them? During the day at low tide, nerites usually hide in crevices and shady spots. At night and on a cool wet day, you might see them creeping about on rocks and boulders, especially those covered in algae.

After having a look at a nerite, be sure to put it back where you found it. And make a big deal about doing this. So visitors get into the habit of doing the same.

Often, if you leave a nerite alone upside down on a rock in a shady spot, it will eventually come out. You can then talk about the living animal and not just the shell.

Too Much Information

What do they eat? Nerites graze the thin layer of algae that grows on rocks.

Nerite babies: Nerites have separate genders and engage in internal fertilisation. So they actually have to make body contact to reproduce. Sometimes you can see mating nerites, especially early in the morning, near wet spots on a rock. The eggs are laid in white circular egg capsules. Each egg capsule may have more than 30 eggs.

What are the different kinds of nerites commonly seen on our shores?
A rough identification can be made by looking at the shell's general shape and texture, and especially the underside of the shell and the teeth-like structures at the shell opening (these do not actually function as teeth to chew food). But similar nerites can only be positively distinguished by looking at internal features of the shell and animal.

Ox-tongue nerite snails (Nerita albicilla)
2-3cm. Shell rounded but flared at the mouth, with faint grooves on the upper surface. On the underside of the shell, on the flat portion in front of the shell opening, there are a series of obvious bumps. This surface resembles the tongue of an ox, hence its scientific name. This nerite is not as commonly seen as the following two.
Chameleon nerite snails (Nerita chamaeleon)
2-3cm. Shell sturdy and rounded with a more flattened spire and rough grooved lines on the upper surface. It has very small 'teeth' near the centre of the shell opening. Some have a row of 4 bumps perpendicular to the edge of the shell opening. It comes in a wide variety of colours and patterns, hence its name. It can be seen in large numbers on some of our rocky shores.Waved nerite snails (Nerita undata)
2-3cm. Shell sturdy and rounded, with a more pointed spire and fine smooth grooved lines on the upper surface. The flat area near the shell opening is white with 3-4 distinct 'teeth'. It comes in a wide variety of colours and patterns. It is found together with Chameleon nerite snails, also in large numbers.Lined nerite snails (Nerita lineata)
2-3cm. Shell sturdy and rounded. It is distinguished by the grey shell with neat black grooved lines. It has small 'teeth' at the shell opening. It is more often seen in and near mangroves. Look for them closer to vegetation and in shady areas nearer the high water mark. The scientific names given here are the old ones. Recent books give different genus and species names to some of the snails. I've yet to get confirmation on the current names.

1 comment:

Tom Eichhorst said...

Great images. Your scientific names are mostly up-to-date, except:

Nerita lineata is Nerita balteata (I can provide more detail if wanted) and your Nerita undata is another species, maybe Nerita aurantia - I cannot be certain from the photos.


Tom Eichhorst